Because I don't think chocolate chicken or barbecue milk are edible. They probably wouldn't kill you, but they definitely wouldn't taste good. Chocolate milk and barbecue chicken are much better.
As you can see, the two sauces look similar, and if you weren't paying attention you could possibly marinate your grilling meat in chocolate sauce. Or stir a big spoonful on barbecue sauce into your glass of milk. BLECH!
Of course I could designate specific color lids for specific sauces. But that would mean I couldn't use those lids for other things, and typically they get used pretty often. It's not uncommon for me to dump some liquid or other into one of my numerous Tupperware cylinders, then go rummaging through the Lid Drawer only to find all my round lids are employed elsewhere (how I ended up with more containers than lids, I'll never know).
Or, alternately, I could label the sauce containers with a piece of tape across the middle and "CHOCOLATE" or "BBQ" in big letters. But, when I've done that in the past, the tape tends to go through the dishwasher with the empty container, and then it is really difficult to remove when I want to put something else in the container.
So I'll probably keep doing what I've been doing and just putting unlabeled sauces into whatever right size cylinder I have, and slapping on whatever round lid I can find in the Lid Drawer. And we'll keep playing Russian Roulette when we just open the fridge and grab in the general direction of whatever sauces are in there.
(Chocolate sauce recipe here, and BBQ sauce recipe here.)